Use this screen to configure the code access security settings that your application should use after deployment.
Select the required trust level for your application. ClickOnce trust levels are defined using XML permissions sets. Trust Levels control what your code will be allowed to do on the users target system, which .Net Framework calls they are able to make.
If you set your application to require full trust, the user will be asked to confirm that they wish to install it. By setting a lower trust level you can prevent this.
Trust Definition shows you the XML that will be included within your application manifest. Selecting Custom trust level allows you to edit the xml in order to define your own trust definition.
The custom definition should contain the XML you wish to appear inside the trustInfo tag within your application manifest file.
See here http://msdn.microsoft.com/en-us/library/6ad1fshk.aspx for more details on the trustInfo tag and valid XML to be contained within it.